Method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation

ABSTRACT

A method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation may include immersing the substrate as an electrode together with a counter-electrode in an alkaline electrolytic aqueous solution. The method may also include the step of applying an electrical potential sufficient to generate spark discharges on the surface of the substrate for a predefined period of treatment time so as to lead to the formation of the coating. The coating may be aluminum oxides and oxides of any alloying agents of the alloy. The electrolytic aqueous solution may have from 9 to 14 g/l of Na2SiO3, from 2.3 to 2.8 g/l of K3PO4, not less than 5 g/l of Na2WO4·2H20, from 0.4 to 1.5 g/l of Na3AlF6, and NaOH at a concentration such that the electrolytic solution has a pH between 11.8 and 12.0, and a conductivity between 9.5 and 10.5 mS/cm.

FIELD OF APPLICATION

The present invention relates to a method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation.

The method according to the invention is applied in particular to aluminum and silicon alloy substrates.

The method according to the invention finds particular application in the automotive field, in the production of protective surface coatings for components of braking systems, since it allows ceramic coatings having high resistance to wear and corrosion to be made.

PRIOR ART

Aluminum alloys, and in particular aluminum-silicon alloys, are widely used in automotive and aerospace applications due to their high strength/density ratio, their machinability and also their excellent castability.

As is known, anodizing is the preferred method for obtaining a corrosion-resistant coating on aluminum alloys.

However, anodizing has some operational limitations.

With anodizing it is difficult to obtain coatings with high thicknesses on aluminum-silicon alloys. This is essentially due to the presence of silicon which tends to inhibit the formation of the anodized coating.

Furthermore, anodizing requires a pickling pre-treatment which greatly affects the fatigue life of the machined aluminum alloy.

As an alternative to anodizing, a process of formation of coatings by means of plasma electrolytic oxidation has been proposed for several years.

Plasma electrolytic oxidation, known as PEO, MAO (Micro Arc Oxidation) or EPO (Electrolytic Plasma Oxidation), is an electrochemical surface treatment which allows different alloys, such as Magnesium, Aluminum and Titanium, to be coated.

The principle underlying plasma electrolytic oxidation is the formation of an oxide layer with dielectric properties on the substrate to be coated. The substrate is immersed as an electrode together with a counter-electrode in an alkaline electrolytic aqueous solution. By applying a sufficient electrical voltage, the treatment works in a discharge regime, creating multiple sparks on the surface. The local temperature of the sparks allows the local remelting of the oxide layer, which reacts with the electrolyte in which the substrate is immersed. The coating layer that is formed has a high adhesive strength, since it penetrates the substrate for a few micrometers, and has a high resistance to corrosion.

There are many applications of the PEO process to aluminum alloys, and in particular the application of Al—Si alloys to aluminum-silicon alloys.

The main problem of the PEO process is the formation of a considerable porous outer layer of low micro-hardness and with numerous micro and macro defects (pores, micro-cracks, flaky patches). The thickness of the defective layer is equal to 25-55% of the total thickness of the ceramic coating, depending on the chemical composition of the substrate and the method of carrying out the electrolysis.

The SEM image of FIG. 3 shows a cross section of a ceramic coating obtained on an aluminum-silicon alloy substrate by means of a traditional PEO process. The lower band represents the aluminum-silicon alloy substrate (indicated with a in the figure); the central band immediately above the substrate (indicated with b1 in the figure) represents the most compact and homogeneous layer of the ceramic coating; the large granular band above the layer b1 (indicated with b2 in the figure) represents the porous surface layer of the ceramic coating; the upper band (indicated by c in the figure) above the ceramic coating b represents the resin layer used to incorporate the sample in order to polish the sample and consequently perform the SEM scan.

Expensive precision equipment is used to remove the porous layer. If the substrate is complex in shape, with surfaces that are difficult to reach for abrasive and diamond tools, the problem of removing the defective layer becomes difficult to solve. This limits the application scope of the process.

This problem has been addressed in particular in patent GB2386907. The PEO process described in such patent allows the quick and efficient formation of uniformly colored wear-resistant, corrosion-resistant, heat-resistant and dielectric ceramic coatings on the surfaces of these articles. The coatings obtained with this process are characterized by a high degree of uniformity of thickness, low surface roughness and by the virtual absence of the aforementioned outer porous layer.

The process described in GB2386907 comprises the following steps: i) supplying the electrodes with bipolar pulses at a high current frequency having a predetermined frequency range (at least 500 Hz); and ii) generating acoustic vibrations in the electrolyte in a predetermined sound frequency range so that the frequency range of the acoustic vibrations overlaps the frequency range of the current pulses. The acoustic vibrations cause the aero-hydrodynamic saturation of the electrolyte with oxygen. For this purpose, the electrolyte is fed with oxygen or air. The process also involves the introduction of ultra-dispersed solid particles into the electrolyte to create a stable hydrosol through acoustic vibrations.

The process described in GB2386907, while leading to appreciable results, is nevertheless complex to control.

Therefore, the need is felt—in particular in the field of braking systems—to have a method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation, which allows ceramic coatings having low roughness, high hardness and high corrosion resistance to be obtained more easily on aluminum alloy substrates.

DISCLOSURE OF THE INVENTION

Therefore, the object of the present invention is to eliminate, or at least reduce, the aforementioned problems relating to the prior art, by providing a method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation which allows ceramic coatings with low roughness, high hardness and high resistance to corrosion to be obtained more easily.

A further object of the present invention is to provide a method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation which allows a coating substantially free of a porous surface layer to be obtained.

A further object of the present invention is to provide a method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation which allows a very homogeneous coating to be obtained.

DESCRIPTION OF THE DRAWINGS

The technical features of the invention are clearly identified in the content of the claims set out below and its advantages will become more readily apparent in the detailed description that follows, made with reference to the accompanying drawings, which represent one or more embodiments provided purely by way of non-limiting examples, in which:

FIG. 1 shows an SEM image of a cross section of a ceramic coating obtained on an aluminum-silicon alloy substrate by means of a traditional PEO process;

FIG. 2 shows the trend of the electrical potential applied to an aluminum-silicon alloy substrate according to a preferred embodiment of the method according to the invention;

FIG. 3 shows an SEM image of a cross section of a ceramic coating obtained on an aluminum-silicon alloy substrate by the method according to the invention; and

FIG. 4 shows an SEM image of the surface of a ceramic coating obtained on an aluminum-silicon alloy substrate by the method according to the invention.

Elements or parts of elements common to the embodiments described hereinafter will be indicated with the same numerical references.

DETAILED DESCRIPTION

The present invention relates to a method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation.

The method according to the invention is generally applied on aluminum alloy substrates, and in particular on aluminum and silicon alloy substrates.

The method according to the invention finds particular application in the automotive FIELD, in the production of protective surface coatings for components of braking systems, since it allows ceramic coatings having high resistance to wear and corrosion to be made.

According to a general embodiment of the invention, the method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation comprises the following steps:

-   -   immersing the substrate as an electrode together with a         counter-electrode in an alkaline electrolytic aqueous solution;     -   applying an electrical potential sufficient to generate spark         discharges on the surface of the substrate for a predefined         period of treatment time so as to lead to the formation of said         coating.

The coating thus obtained consists mainly of aluminum oxides and oxides of any alloying agents of said alloy.

In particular, the aforesaid substrate is made of aluminum and silicon alloy, and even more particularly of aluminum alloy with a high silicon content (>7% by weight). In this case, the ceramic coating obtained is a layer consisting mainly of a mixture of aluminum oxides, silicon oxides and mixed aluminum-silicon oxides.

Advantageously, the substrate consists of a component of a braking system, in particular of a disc braking system. Preferably, the substrate consists of a brake caliper, a brake caliper piston or a brake disc bell.

In particular, the method according to the invention is of the electrolytic type. In particular, such method comprises an anode, represented by the substrate, and a counter-electrode.

In particular, the method according to the invention comprises a counter-electrode which may be a secondary electrode. Alternatively, such counter-electrode is represented by a cathode. Alternatively, such counter-electrode is represented by the container containing the alkaline electrolytic aqueous solution.

According to the invention, the electrolytic aqueous solution comprises:

-   -   from 9 to 14 g/l of Na2SiO3;     -   from 2.2 to 2.8 g/l of K3PO4;     -   not less than 5 g/l of Na2WO4·2H20;     -   from 0.4 to 1.5 g/l of Na3AlF6; and     -   NaOH at a concentration such that the electrolytic solution has         a pH between 11.8 and 12.0, and a conductivity between 9.5 and         10.5 mS/cm.

Preferably, the electrolytic aqueous solution contains only the electrolytes indicated above: Na2SiO3; K3PO4; Na2WO4·2H20; Na3AlF6; NaOH.

It has been experimentally verified that, using alkaline electrolytic solutions having the above composition, the ceramic coatings obtainable on aluminum alloy substrates by means of plasma electrolytic oxidation have the following features:

-   -   high resistance to corrosion;     -   high hardness (HV0.01<1,400);     -   low surface roughness (Ra<2 μm);     -   high morphological homogeneity of the coating layer;     -   porous surface layer absent or substantially absent, or at least         having a thickness not exceeding 5% of the total thickness of         the ceramic coating.

Such results were obtained using the electrolytic solution described above in a traditional PEO process, and then setting the usual electrical process parameters, such as applied electrical potential, current density, frequency and duration of the plasma discharge process.

The method according to the invention therefore does not require particular adjustments of the electrical process parameters, nor the adoption of particular control methods of the PEO process.

In particular, as will be resumed hereafter, the method according to the invention may be advantageously applied in conditions of low energy consumption, without particular limitations to the speed of formation of the ceramic coating on the substrate.

It has been experimentally verified that the sodium silicate (Na2SiO3) present in the electrolytic solution (in the concentrations indicated above) contributes to significantly improving the density of the ceramic coatings obtained through the PEO process and therefore, consequently, the resistance to corrosion.

It has also been experimentally verified that the absence of Na2SiO3 in the electrolytic solution inhibits the correct growth of the coating and in most cases the plasma discharges do not start correctly.

As already mentioned, the alkaline electrolytic solution contains from 9 to 14 g/l of Na2SiO3. Preferably, the electrolytic solution comprises from 9 to 11 g/l of Na2SiO3, and more preferably 10 g/l.

With such concentrations of Na2SiO3 a compromise was surprisingly found between a high growth rate of the coating and a reduction in surface roughness. In fact, it has been observed that concentrations lower than the values indicated above (2, 5, 3, 5, 8 g/l of Na2SiO3) have determined an insufficient growth rate of the coating, while higher concentrations (15 or 20 g/l of Na2SiO3) resulted in a higher growth rate of the coating (and therefore a greater thickness), associated however with a considerable increase in the aggressiveness of the process on the substrate and in the roughness of the coatings. In fact, at higher concentrations, more heterogeneous and less dense coatings were obtained, with a worsening of corrosion resistance.

Advantageously, it was also possible to verify that Na2SiO3 may be introduced into the electrolytic solution both in solid powder and already in solution.

As already mentioned, the alkaline electrolytic solution contains from 2.2 to 2.8 g/l of K3PO4. Preferably, the electrolytic solution comprises from 2.4 to 2.6 g/l of K3PO4, and more preferably 2.5 g/l.

It has been experimentally verified that the potassium phosphate (K3PO4) present in the electrolytic solution further contributes to improving the density of the ceramic coatings obtained through the PEO process and therefore, consequently, the resistance to corrosion.

Electrolytic solutions containing K2HPO4 or KH2PO4 in place of K3PO4 were tested. In both cases the results were poor. A white precipitate (composed mainly of phosphorus) was detected on the coatings obtained.

As already mentioned, the alkaline electrolytic solution contains not less than 5 g/l of Na2WO4·2H20.

It has been experimentally verified that sodium tungstate dihydrate (Na2WO4·2H2O) significantly increases the ratio of the thickness of the dense layer to the total thickness of the coating (see FIG. 5 ). The presence of such component in the electrolytic solution leads to an increase in the hardness of the ceramic coatings obtained. This is attributable to the fact that the ceramic coatings obtained contain tungsten.

Advantageously, it is possible to use electrolytic solutions with higher concentrations of sodium tungstate dihydrate (Na2WO4·2H2O), in order to further increase the hardness of the coatings obtained.

Preferably, the electrolytic solution comprises 5 g/l of Na2WO4·2H20. With this concentration, in fact, already satisfactory results have been obtained.

As already mentioned, the alkaline electrolytic solution contains from 0.4 to 1.5 g/l of Na3AlF6.

It has been experimentally verified that sodium hexafluoroaluminate (Na3AlF6) allows the surface roughness of the ceramic coatings obtained to be decreased.

Preferably, the electrolytic solution comprises from 0.4 to 0.6 g/l of Na3AlF6, more preferably 0.5 g/l. In fact, with these concentrations, coatings with higher morphological homogeneity were obtained.

As already mentioned, the alkaline electrolytic solution comprises NaOH at a concentration such that the electrolytic solution has a pH between 11.8 and 12.0, and a conductivity between 9.5 and 10.5 mS/cm.

Preferably, the alkaline electrolytic solution comprises NaOH in a concentration such that the alkaline electrolytic solution has a pH of 11.9, and a conductivity of 10.0 mS/cm.

Preferably, the electrolytic solution comprises from to 1.2 g/l of NaOH, more preferably from 0.9 to 1.1 g/l, and even more preferably 1.0 g/l.

Sodium hydroxide (NaOH) is introduced into the solution to bring the pH and conductivity of the electrolytic solution to the values indicated above.

Surprisingly, it was found that sodium hydroxide (NaOH), unlike potassium hydroxide (KOH), which could be used as an alternative to adjust the pH of the solution, allows plasma discharges to start without leading to an excessive dissolution of the substrate to be coated, also leading to more homogeneous coatings.

The replacement of NaOH with KOH has in fact been studied, but obtaining negative results, such as excessive dissolution of the metal substrate without starting the plasma, very aggressive processes or heterogeneous coatings. Furthermore, the degree of dispersion of the electrolyte was also altered, forming a white precipitate.

The pH and electrical conductivity values depend on the chemical composition of the electrolytic solution. The electrical conductivity of the electrolytic solution plays a more relevant role as it is directly related to the final and stable voltage value of the PEO process which will determine, among other things, the energy consumption of the process.

For dilute electrolytic solutions, the final voltage value will be very high, while concentrated electrolytic solutions may lead to excessive dissolution of the metal substrate, preventing plasma initiation and inhibiting the growth rate of the coating.

Similar pH and conductivity values to those indicated above could be obtained by modifying the chemical composition and concentration of the electrolytic reagents. However, changing the composition and concentrations of the electrolytic solution would alter the morphology, composition and properties of the PEO coatings obtained.

According to a preferred embodiment of the method according to the invention, the electrolytic solution has a pH equal to 11.9 and a conductivity equal to 10.0 mS/cm and comprises:

-   -   10 g/l of Na2SiO3;     -   2.5 g/l of K3PO4;     -   5 g/l of Na2WO4·2H20;     -   0.5 g/l of Na3AlF6;     -   1.0 g/l NaOH.

Preferably, during the PEO process the alkaline electrolytic aqueous solution is cooled by means of a cooling system, preferably to maintain said alkaline electrolytic aqueous solution at a temperature between 25° C. and 45° C. during said predefined period of treatment time.

As already mentioned above, the ceramic coatings described above were obtained using the electrolytic solution described above in a traditional PEO process, and then setting the usual electrical process parameters, such as applied electrical potential, current density, frequency and duration of the plasma discharge process.

The method according to the invention therefore does not require particular adjustments of the electrical process parameters, nor the adoption of particular control methods of the PEO process.

Preferably, the electrical potential is kept substantially constant for the aforementioned predefined period of time, preferably at a value between 300 and 400 V, more preferably equal to 350V, as illustrated in FIG. 2 .

Preferably, during the aforementioned predefined period of treatment time an electrical current with a current density between 20 and 25 A/dm3, preferably 25 A/dm3, is applied to the substrate.

Preferably, an electrical current having a frequency of at least 50 Hz, even more preferably equal to 50 Hz, is applied to the substrate. It has been experimentally verified that by using the electrolytic solution described above already at frequencies of 50 Hz, very dense coatings (substantially free of porosity) are obtained, without the need to increase the frequency of the current.

Preferably, the electrical current is applied continuously (not pulsed). However, the current may also be applied in a pulsed mode.

Preferably, the predefined period of treatment time is comprised between 20 and 40 min, more preferably equal to 30 min.

According to a preferred embodiment of the invention, the electrical potential is kept substantially constant for said predefined period of treatment time at a value of 350V. During a period of treatment time of 30 min, an electrical current with a current density of 25 A/dm3 and a frequency of 50 Hz is continuously applied to the substrate.

As already mentioned, the method according to the invention may therefore be advantageously applied in conditions of low energy consumption, without particular limitations to the speed of formation of the ceramic coating on the substrate.

Advantageously, coating formation rates of between and 1 μm/min were observed.

Advantageously, the method for producing a ceramic coating on the surface of an aluminum alloy substrate according to the invention may comprise a step c) of pre-treatment of the substrate, to be carried out before said steps a) and b).

Such pre-treatment step c) consists in subjecting the substrate to caustic attack and subsequently in washing the substrate itself with distilled water.

Preferably, the aforementioned caustic attack is obtained by immersing the substrate for a predefined period of time in an aqueous solution of NaOH, preferably containing 50 g/l of NaOH, maintained at a temperature between 60° C. and 70° C., preferably at 60° C. The aforesaid predefined immersion time being comprised between 5 and 15 min, preferably equal to 10 min. It has been experimentally found that the above NaOH concentration allows an adequate pickling of the aluminum alloy substrate to be carried out, reducing the release of A13+ ions from the substrate (and therefore the aggressiveness on the substrate itself) and the formation of slurry in the caustic bath.

It has been verified that the caustic attack (and subsequent washing) of the substrate leads to an improvement in the aesthetic finish of the final ceramic coating obtained, in terms of greater homogeneity of the coating.

However, the caustic attack may also not be carried out if the substrate already has a clean surface, substantially free of dirt and impurities.

Advantageously, in the aforesaid step c) of pre-treatment of the substrate, after the caustic attack and subsequent washing with distilled water, the substrate may be immersed in an acid bath for a predefined period of time and then washed with distilled water.

Preferably, the aforementioned acid bath consists of an aqueous solution of nitric acid. The predefined time of immersion in said acid bath is between 5 and 15 s, preferably equal to 10 s.

Operatively, the immersion in an acid bath is carried out to have a desmutting treatment of the substrate after the caustic attack.

Advantageously, the method for producing a ceramic coating on the surface of an aluminum alloy substrate according to the invention does not require specific post-treatment steps after the aforementioned steps a) and b), i.e. at the end of the PEO process.

In particular, there is no need for post-treatments to seal the surface porosity in order to guarantee the anti-corrosion properties.

However, the method may comprise a post-treatment step d) of the substrate, to be carried out after said steps a) and b), wherein said post-treatment consists of: —washing said substrate with distilled water; —cleaning the surface of said substrate with alcohol; and—allowing it to dry at room temperature.

The ceramic coatings obtained on aluminum alloy substrates with the method according to the invention essentially consist of a compact non-porous layer, which may possibly present on the surface a porous layer having a thickness not exceeding 5% of the total thickness of said coating.

Preferably, as illustrated in the SEM image of FIG. 3 , the ceramic coatings obtained on aluminum alloy substrates with the method according to the invention consist only of a compact non-porous layer.

More in detail, in the SEM image of FIG. 3 the darker band at the bottom represents the aluminum-silicon alloy substrate (indicated with a in the figure); the central band represents the PEO ceramic coating (indicated with b in the figure); the upper band of variegated color (indicated with c in the figure) above the ceramic coating represents the resin layer used to incorporate the sample in order to perform the SEM scan. From this SEM image it may be seen that the ceramic coating obtained consists of a single homogeneous and compact layer, substantially free of porosity. The porous surface layer that is normally present in traditional coatings obtained with PEO processes (visible instead in FIG. 1 ) is practically absent.

The ceramic coatings obtained on aluminum alloy substrates with the method according to the invention have roughness Ra≤2 μm and hardness HV0.01>1,400.

From an aesthetic point of view, the ceramic coatings obtained on aluminum-silicon alloy substrates with the method according to the invention have a very homogeneous dark gray surface color.

These features may be seen from the SEM image of FIG. 4 which shows the surface of a ceramic coating obtained on an aluminum-silicon alloy substrate by means of the method according to the invention.

This result (color homogeneity) confirms the fact that by the method according to the invention it is possible in a homogeneous manner aluminum alloys with a high silicon content, which are not easy to cover homogeneously. In fact, if a standard anodizing process were used as an alternative, some defects could be detected in areas with a high silicon content.

An example of application of the method according to the invention for coating a substrate consisting of a disc brake caliper made of an aluminum alloy with a high silicon content is given below.

More specifically, the caliper is made with an Al/Si7%/Mg/Ti aluminum-silicon alloy.

An electrolytic aqueous solution was used having a pH equal to 11.9 and a conductivity equal to 10.0 mS/cm and comprising: —10 g/l of Na2SiO3; —2.5 g/l of K3PO4; —5 g/l of Na2WO4·2H20; —0.5 g/l of Na3AlF6; —1.0 g/l of NaOH.

The substrate was immersed as an electrode together with a counter-electrode in the aforementioned electrolytic aqueous solution, thereby applying an electrical potential sufficient to generate spark discharges on the surface of the substrate for a period of treatment time of 30 min to lead to the formation of the coating. As illustrated in FIG. 2 , the electrical potential was kept substantially constant for the period of treatment time at a value of 350V. During such period of time, an electrical current with a current density of 25 A/dm3 and a frequency of 50 Hz is continuously applied to the substrate.

During the treatment, the alkaline electrolytic aqueous solution was cooled by means of a cooling system to a temperature between 25° C. and 45° C.

At the end of the treatment, a ceramic coating was obtained having an average thickness of about 30 μm, consisting of a mixture of aluminum oxide, silicon oxide and aluminum silicate oxide. Sodium, Potassium, Phosphorus, Tungsten are present in traces (<2 atomic %). The elemental composition of the coating obtained from an EDS spectroscopy analysis is reported below in Table 1.

TABLE 1 Element % by weight atomic % C 6.70 12.14 O 42.69 58.09 Na 0.39 0.37 Al 15.64 12.62 Si 17.21 13.34 P 1.53 1.07 K 0.82 0.46 Ca 0.30 0.16 W 14.75 1.74

The ceramic coating penetrates into the substrate for a few micrometers. This feature ensures adhesion of the coating to the substrate.

The SEM images of FIGS. 3 and 4 refer to the ceramic coating obtained on the caliper treated according to this example. The ceramic coating consists only of a compact non-porous layer. The coating obtained has a dark gray and very homogeneous surface color.

The ceramic coating has a surface roughness Ra<2 m and a hardness HV0.01>1,400.

Obtaining such a low roughness value is important in the application on the brake caliper, and in particular inside the caliper piston seat, to reduce the risk of abrasion of the caliper body on the piston.

Similarly, obtaining such high hardness values (difficult to obtain with standard anodizing) is important for moving mechanical components.

The so-coated brake caliper was subjected to a corrosion resistance test using an NSS (neutral salt spray) test. The caliper passed the test, reporting a minimum resistance of 480 h without any damage to the substrate (black holes).

The invention allows numerous advantages to be obtained which have been explained in the course of the description.

The method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation according to the invention allows ceramic coatings having low roughness, high hardness and high corrosion resistance to be obtained more easily.

The method according to the invention allows a coating substantially devoid of a porous surface layer and very homogeneous to be obtained.

The method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation according to the invention allows in particular a ceramic coating to be obtained on an automotive component having:

-   -   a smooth surface,     -   high wear resistance;     -   high resistance to corrosion;     -   homogeneous morphology, substantially devoid of a porous surface         layer;     -   high aesthetic features (homogeneous coloring)

These features are also associated with coating thicknesses not exceeding 50 μm.

The invention thus conceived therefore achieves its intended purposes.

Of course, in its practical implementation it may also assume different forms and configurations from the one illustrated above, without thereby departing from the present scope of protection.

Furthermore, all details may be replaced with technically equivalent elements, and dimensions, shapes and materials used may be any according to the needs. 

1-25. (canceled)
 26. Method for producing a ceramic coating on the surface of an aluminium alloy substrate by means of electrolytic plasma oxidation, comprising the following steps: a) immerging the substrate as an electrode together with a counter-electrode in an alkaline electrolytic aqueous solution; b) applying an electrical potential sufficient to generate spark discharges on the surface of the substrate for a predefined period of treatment time so as to lead to the formation of said coating, consisting mainly of aluminium oxides and oxides of any alloying elements of said alloy, wherein the electrolytic aqueous solution comprises: from 9 to 14 g/l of Na2SiO3; from 2.3 to 2.8 g/l of K3PO4; not less than 5 g/l of Na2WO4·2H20; from 0.4 to 1.5 g/l of Na3AlF6; NaOH at a concentration such that the electrolytic solution has a pH between 11.8 and 12.0, and a conductivity between 9.5 and 10.5 mS/cm.
 27. The method according to claim 26, wherein the electrolytic solution comprises from 9 to 11 g/l of Na2SiO3, preferably 10 g/I.
 28. The method according to claim 26, wherein the electrolytic solution comprises from 2.4 to 2.6 g/l of K3PO4, preferably 2.5 g/I.
 29. The method according to claim 26, wherein the electrolytic solution comprises 5 g/l of Na2WO4·2H20.
 30. The method according to claim 26, wherein the electrolytic solution comprises 0.4 to 0.6 g/I Na3AlF6, preferably 0.5 g/I.
 31. The method according to claim 26, wherein the electrolytic solution comprises NaOH at a concentration such that the electrolytic solution has a pH of 11.9, and a conductivity of 10.0 mS/cm.
 32. The method according to claim 26, wherein the electrolytic solution comprises 0.8 to 1.2 g/I NaOH, preferably 0.9 to 1.1 g/I, even more preferably 1.0 g/I.
 33. The method according to claim 26, wherein the electrolytic aqueous solution comprises: 10 g/l of Na2SiO3; 2.5 g/l of K3PO4; 5 g/l of Na2WO4·2H20; 0.5 g/l of Na3AlF6; 1.0 g/I NaOH, and wherein the electrolytic solution has a pH of 11.9 and a conductivity of 10.0 mS/cm.
 34. The method according to claim 26, wherein the alkaline electrolytic aqueous solution is cooled by means of a cooling system, preferably to maintain said alkaline electrolytic aqueous solution at a temperature between 25° C. and 45° C. during said predefined period of treatment time.
 35. The method according to claim 26, wherein the electrical potential is kept substantially constant for said predefined period of time, preferably at a value between 300 and 400 V, more preferably 350 V.
 36. The method according to claim 26, wherein during said predefined period of treatment time an electrical current with a current density between 20 and 25 A/dm3, preferably 25 A/dm3, is applied to the substrate.
 37. The method according to claim 26, wherein an electrical current with a frequency of at least 50 Hz, preferably 50 Hz is applied to the substrate.
 38. The method according to claim 36, wherein the electrical current can be applied continuously or in a pulsed mode.
 39. The method according to claim 26, wherein the predefined period of treatment time is between 20 and 40 min, preferably 30 min.
 40. The method according to claim 26, wherein the electrical potential is kept substantially constant for said predefined period of treatment time at a value of 350V and wherein during said predefined period of treatment time an electrical current with a current density equal to 25 A/dm3 and a frequency equal to 50 Hz is applied continuously to the substrate, said predefined period of treatment time being 30 min.
 41. The method according to claim 26, comprising a pre-treatment step (c) of the substrate, to be carried out before said steps (a) and (b), wherein the pre-treatment consists of subjecting said substrate to caustic attack and then washing said substrate with distilled water.
 42. The method according to claim 41, wherein said caustic attack is obtained by immerging said substrate for a predefined period of time in an aqueous solution of NaOH, preferably containing 50 g/l of NaOH, maintained at a temperature between 60° C. and 70° C., preferably at 60° C., said predefined immersion time being between 5 and 15 min, preferably 10 min.
 43. The method according to claim 41, wherein in said step (c) of pre-treatment of the substrate, after the caustic attack and subsequent washing with distilled water, the substrate is immersed in an acid bath for a predefined period of time and then washed with distilled water.
 44. The method according to claim 43, wherein said acid bath consists of an aqueous solution of nitric acid, said predefined immersion time in said acid bath being between 5 and 15 s, preferably 10 s.
 45. The method according to claim 26, comprising a post-treatment step (d) of the substrate, to be carried out after said steps (a) and (b), wherein said post-treatment consists of: washing said substrate with distilled water; cleaning the surface of said substrate with alcohol; and allowing it to dry at room temperature.
 46. The method according to claim 26, wherein said ceramic coating consists essentially of a non-porous, compact layer, which may possibly have on the surface, a porous layer with a thickness not exceeding 5% of the total thickness of said coating.
 47. The method according to claim 46, wherein said ceramic coating consists only of said non-porous, compact layer.
 48. The method according to claim 46, wherein said ceramic coating has a roughness Ra≤2 μm and a hardness HV0.01≥1.400.
 49. The method according to claim 26, wherein said substrate is in aluminium-silicon alloy and wherein the ceramic coating obtained is a layer consisting mainly of a mixture of aluminium oxides, silicon oxides and mixed aluminium-silicon oxides.
 50. The method according to claim 26, wherein said substrate consists of a component of a braking system, preferably a disc braking system, in particular a brake caliper, a brake caliper piston or a brake disc bell. 